Point-To-Point-Based Centralized Protection and Control System Design For A Two-Transformer Distribution Substation

Duke Energy has expanded the utilization of fiber optics in various aspects such as protection, control, alarming, and communication schemes. However, copper cables continue to be used in connecting current transformers (CTs) and potential transformers (PTs) to protective relays and metering devices. The viability of deploying a point-to-point digital secondary system (P2P DSS) to replace these copper cables in substations is being explored. Furthermore, Duke Energy is also investigating centralized protection and control (CPC) technology to reduce the number of devices in the substation. Fewer devices translate to fewer relay models for installation, commissioning, configuration, testing, and inventory management. As the CPC system has access to analog and binary signals from throughout the substation, its event record simplifies disturbance analysis. Recent efforts have explored the potential use of a P2P-based CPC system for its simplicity, potential cost savings in substation construction, and reduced construction time.

An existing two-transformer distribution substation was selected as the basis for the P2P-based CPC system design. The substation consists of two delta-wye-grounded step-down transformers, tapped from 100 kV parallel transmission lines. A single circuit breaker feeds the high-voltage side of both transformers. Each transformer (20 MVA) steps down the voltage to 12.5 kV and connects to the low-voltage bus. Motor-operated ganged disconnects are installed on the high-voltage side of the transformer, and circuit breakers are installed on the low-voltage side. Each transformer feeds two distribution feeders. A bus-tie breaker is installed between two low-voltage buses. The tapped point is protected using an overcurrent relay. Each transformer is protected using a high-side overcurrent relay, a transformer bank differential, a low-side bus differential, and a low-side overcurrent relay. Each distribution feeder is protected using an overcurrent relay. The existing substation employs 13 relays of 4 distinct types for overall protection and control.

This paper outlines the detailed design of a simple P2P-based CPC system tailored for a two-transformer distribution substation. The design utilizes P2P MUs and a P2P CPC system to protect and control the entire substation. Each CPC protects one transformer, one low-voltage bus, and their associated feeders. To eliminate a single point of failure, two CPCs are used for each transformer. When redundant MUs are not considered, the new design uses 10 MUs and 4 CPCs. However, with redundant MUs, a total of 18 MUs and 4 CPCs are required. The P2P MUs do not have any settings. As a result, in the new design, only CPCs require configuration, as compared to 13 relays in the existing substation. The paper includes test results that demonstrate the performance of the new CPC system for transformer and bus differential protection and compares the results with the existing relays. Furthermore, the paper delves into Duke Energy’s perspective, exploring issues of potential benefits, design questions, and technical challenges. This paper also discusses the utility’s perspective on the concerns and risks associated with the level of centralization, potential failure modes, and change management requirements for engineering and testing with a centralized design.